Protective surface coatings are commonly known as paint coatings. Clear and pigmented surface coatings are organic compositions applied to substrates to form continuous films which are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrate. Pigmented protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other additives. The polymeric binder functions as a vehicle for the pigments, inerts, and other additives in wet coating compositions and further functions as a crosslinked binder for the pigments and inert fillers in the cured or hardened paint film. In aqueous coating systems, the prime pigments and inert fillers are normally dispersed in water with wetting and dispersing agents that are suited for the purpose. A protective colloid (e.g. hydroxyethyl cellulose, polyvinyl alcohol, etc.) is usually added to provide a steric stabilization for the dispersed pigments. The aqueous pigment dispersion is then combined with waterborne polymers that bind the pigments and inert fillers in the cured or hardened paint film. The thermosetting polymeric binders of this invention comprise thermosetting functional polymers coreactive with carbodiimide crosslinkers. Carboxyl functional latex resins crosslinked by polycarbodiimides are disclosed in European Patent Application Publication No. 0,121,083 published Oct. 10, 1984 based on Application No. 84101921.9 filed Feb. 23, 1984. Water-dispersed polyurethanes are disclosed in U.S. Pat. Nos. 4,335,029; 3,826,769; 4,305,858; 4,306,998; 4,147,680; 4,211,683; and 4,408,008.
It now has been found that aqueous thermosetting coatings based on water-dispersed polyurethane and emulsion polymeric acrylic polymers can be adapted to be crosslinked at ambient temperatures with carbodiimide crosslinkers where such thermosetting compositions are particularly suitable for application and adhesion to temperature sensitive plastic substrates. The cured paint films exhibit excellent flexibility and adhesion as well as solvent and water resistance over several types of plastic substrates. The waterborne polymers of this invention include emulsion copolymers and polyurethane dispersions that form a continuous film via a coalescence and thermosetting process at ambient or mild force dry conditions. In these systems, the filming process is initiated by the evaporation of water. Film properties are further enhanced by the post addition of a carbodiimide crosslinker that is coreactive with carboxyl functional sites on the emulsion copolymer and polyurethane polymers. The reaction between carbodiimides and carboxyl functional polymers to form polymeric N-acyl ureas occurs at room temperature in most systems, or at mild bake temperatures that are well below the heat distortion temperatures of most plastics in others. The water-dispersed polyurethane is included in the polymeric mixture to enhance the hardness/flexibility and resistance properties of the coating. The emulsion copolymer provides good adhesion to a wide variety of plastic substrates. Both polymers contain functional groups that react with carbodiimide crosslinkers under forced dry or ambient conditions to provide further enhancement of resistance properties. Waterborne clear and pigmented coating systems that utilize this polymer matrix as a binder have been developed. The polymer matrix is particularly useful in primer coatings for plastic substrates. These and other advantages of this invention will become apparent by referring to the detailed description and illustrative examples of the invention.